Modern communications networks are increasingly based on silica optical fiber which offers very wide bandwidth within several spectral wavelength bands. In recent developments, the transmission capacity of fiber optic systems has been greatly increased by wavelength division multiplexing (WDM), wherein multiple independent optical signals, differing uniquely by wavelength, are simultaneously transmitted over the fiber optic link. For example, the C-band transmission window has a bandwidth of about 35 nanometers, determined partly by the spectral amplification bandwidth of an erbium-doped fiber amplifier (EDFA) amplifier, in which multiple wavelengths may be simultaneously transmitted. All else being equal, for a WDM network containing N number of wavelengths, the data transmission capacity of the link is increased by a factor of N. Dense WDM (DWDM) systems are being designed wherein the transmission spectrum includes 40, 80, or more wavelengths with wavelength spacing of less than 1 nanometer. Current designs have wavelength spacing of between 0.4 and 0.8 nanometer or equivalently a frequency spacing of 50 to 100 GHz respectively.
In a modern fiber optic switch of a sophisticated wavelength switching all-optical network, the wavelength components W from an incoming multi-wavelength fiber are de-multiplexed into different spatial paths. Switching elements then route the wavelength-separated signals toward the desired output fiber port before a multiplexer aggregates the optical signals of differing wavelengths onto a single outgoing fiber. In conventional fiber switching systems, all the fiber optic switching elements and associated multiplexers and de-multiplexers are incorporated into a wavelength selective switch (WSS), a specially enhanced optical cross connect (OXC) having a dispersive element and wavelength-selective capability. Additionally, such systems incorporate lenses and mirrors to focus and reflect light, and lenslets which collimate such light.
Input and output optical fibers coupled to the fiber optic switch may be bundled or coupled and concentrated in a fiber port array to secure multiple fibers in a selected position and/or orientation. While the term optical fiber will henceforth be used exclusively with reference to the means of conducting an optical signal to and from the fiber port array, it should be understood that optical fiber(s), waveguide(s), or combination thereof may be implemented to provide an optical input signal to a free-space interface, and to receive an optical output signal therefrom. Typically, each of the optical fibers is substantially aligned parallel with the others, defining a switching plane. Furthermore, each of optical fibers comprises a termination point defining an interface with free-space, wherein optical signals propagating within an optical fiber and/or a waveguide within the fiber port array may exit the fiber and/or waveguide, and propagate through free-space. Internally within said fiber port array are waveguides, utilized to bring the respective optical signals of said fibers closer together on an output face on the other end of the fiber port array in an effort to send the optical signals into, and receive them from free space in a close configuration.
This close configuration of input and output fibers in the switching plane, combined with reflective fiber optic switching elements, often results in the introduction of static back reflection (return loss) and in-to-in crosstalk (coupling) into an optical switch. Back reflection is a measure of optical signal reflected by a fiber optic switching element from an optical fiber back towards the source, into the same optical fiber. In-to-in crosstalk refers to a switching element configuration intending one coupling outcome between optical fibers but further creating an undesired coupling effect between one or more other input optical fibers.
Moreover, fiber optic transmission systems use lasers and amplifiers to transmit signals over optical fiber. These components can be sensitive to light returning into them. A high back reflection or in-to-in crosstalk can prevent such a laser from transmitting correctly.
Prior art for preventing the introduction of static back reflection and in-to-in crosstalk into an optical switch relied upon introduction of an optical isolator in each optical fiber path. An optical isolator is an optical component which allows the transmission of light in only one direction, thus, preventing unwanted feedback into the fiber, such as back reflection, return loss and crosstalk. However, adding optical isolators to each fiber in a multi-fiber switch adds additional cost, installation time, and insertion loss (the measure of power lost due to imperfections in an optical communication link due to discontinuities, such as splicing and junctions required to insert the isolator), especially as the port count of optical switches increases.
Additional prior art for resolving dynamic back reflection or in-to-in crosstalk includes optical switches utilizing dual-axis tilting mirrors to eliminate momentarily produced dynamic crosstalk. For example, when an optical switch selectively connects an optical signal from a selected input port to a selected output port, the most apparent method of readjusting the mirrors would change the mirror angles about the switching axis, whereby any optical power emanating from the input port's waveguide is swept along a line from the previously selected output waveguide to the new selected output waveguide. However, at some time during the sweep, the optical power couples into the intermediate waveguide(s) that lie therebetween, but are not involved in either of the connections. Optical power spuriously induced in the intermediate waveguide(s) momentarily produces crosstalk in the intermediate waveguide(s). A prior art solution to dynamic crosstalk resulting from waveguides being disposed along a line on the output face of the fiber port array is to utilize dual-axis tilting mirrors to cause the switching beam to follow an offset path involving first a cross-axis tilt in the wavelength direction away from the line containing waveguide faces, a second switching axis tilt in the fiber direction, and a third cross-axis tilt back toward the waveguide face, to steer the beam to become coincident with the desired output waveguide while avoiding the intermediate waveguide(s).
Nonetheless, it is clear that there is an unmet need for a system and method for an improved optical fiber/waveguide arrangement that functions to reduce static back reflection, crosstalk and other stray light, but does not impose the cost, complexity, and insertion loss penalties brought about by additional components.